(1) Field of the Invention
The present invention is directed to towed arrays of hydrophones. In particular, the present invention is directed to a method for mitigating spatial aliasing that takes advantage of the forward motion of towed array elements (hydrophones) to overcome the frequency constraints imposed on beam forming by the spatial separation of the array elements.
(2) Description of the Prior Art
The Nyquist criterion describes the sampling rate required for a given frequency without aliasing. Similarly, if an array of sensors (hydrophones) is coherently processed to determine the direction of an incoming acoustic signal through beam forming, then the separation of the array elements determines the maximum frequency that can be processed without producing an alias in direction. If the beam forming limits are exceeded, however, spatial aliasing occurs in the form of grating lobes as graphically illustrated that interfere with the interpretation of the direction of the incoming signal. Grating lobes appear at an aliased look direction that is a function of both source direction and frequency, so a loud, multi-tonal signal will produce grating lobes that are present at multiple look directions, interfering with the interpretation of the number of signals present. Grating lobes are caused by spatial under sampling. What is needed is a method for mitigating spatial aliasing to avoid grating lobes.